Question about PID temperature controller

[davosg]

Hi !

Right now I'm designing a PID controller for the TEC of a laser diode. As you can see at the beginning (right side) I have a Wheatstone bridge, so I can obtain the laser temperature. The potentiometer R4 is simulating the thermistor of the laser and the potentiometer and R3 is used to set the desired temperature.

My problem is that when I simulate, having as initial condition the R4 different of R3, it doesn't stabilizes.

For example, simulation starts with R3=R4=10K and PID giving 85mV. I change R4 to 7K and the PID gives 1.8V, later I return R4 to 10K the PID stabilizes and returns to 85mV. Everything's good here.

When I start the simulation having R3=10K and R4=7K, it gives an output of 1V, as expected, but when I change R4 to 10K, it doesn't stabilizes, it keeps going, but when I return R4 to 7K, it "stabilizes". Looks like the setpoint isn't 10K, is 7K. 

If I didn't explain myself well, I attached some images with the behaviour of the PID output indicating the values of R3 and R4. Also I attached the MS12 file. 

I don't know if it's the simulation because the behaviour of the potentiometer is different than the thermistor or I have some K values wrong. 

If you have any suggestion, you're very welcomed to reply. Thank you! 🙂